I have a 25Mbps Comcast (cable) connection that gives me 30Mbps throughput, measured with active torrents (not while playing YouTube videos obviously). When viewing YouTube videos, they pause → buffer → play → pause → buffer etc etc. This happens on our wired PC, wired iMac, wireless MacBook, iPad and iPhone. Whether I have 1 video playing or 20, they all play the same speed/rate it seems. Never has a single YouTube video even put a dent in my bandwidth utilization (measured at the router) so I know it’s more than likely not my connection.

Probably unrelated, but I get 20-25ms latency to www.youtube.com with 0 packet loss. Here is my Speedtest.net test:

Is there a way to force YouTube to utilize more of my unused bandwidth to reduce the pauses and provide a better viewing experience?

This is an opt-in beta for "Feather" support on YouTube. The "Feather"
project is intended to serve YouTube video watch pages with the lowest
latency possible. It achieves this by severely limiting the features
available to the viewer and making use of advanced web techniques for
reducing the total amount of bytes downloaded by the browser. It is a
work in progress and may not work for all videos.

You can't - not directly, anyway. YouTube will automatically adjust the playback quality depending on your connection to the YouTube servers. Even if your overall connection seems good, your connection to YouTube in particular may not be great - usually this is a problem with the ISP.

There's a caveat here, though. Judging from your speedtest.net image, you are using ComCast. They are well known for using their own intermediate caching servers for YouTube, presumably to decrease their own costs. You're not actually connecting to YouTube, you're connecting to ComCast's servers - which are known to cause playback quality issues.

This thread has a number of methods to bypass these caching servers, depending on your operating system. I'm not sure if this still works the same (and since I don't use ComCast, I can't test it), but it should at least provide a starting point for solving the problem.